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Katheine Heigl nearly brought herself to tears on "The Tonight Show with Jay Leno" on Thursday night, when she opened up to the host about finally understanding – from a mother's perspective – the loss of her older brother, Jason.

Katherine has spoken about the loss of her brother before, who died while she was young, the result of a tragic car accident, but her daughter Naleigh, whom she and husband Josh Kelley adopted last year, made her feel his loss from another perspective.

"I had said to my mother, and I don't mean to get maudlin, my brother…died over 20-something years ago in a car accident and I was really little and … it affected my family a lot," Katherine said, choking up as she spoke to Jay. "I could never have understood what she went through and what that must have been like until I held this child [Naleigh] in my hand and in my arms. The overwhelming need to keep her safe is just instinct."

Katherine told Jay she has been getting very emotional since she first set eyes on her baby girl.

"Initially with her, I obviously loved her, I was very protective of her, I felt like it was my duty, my job to keep this child safe and happy and sheltered, but that duty starts to turn into this feeling that is overpowering and overwhelming and I don't' know how to explain it," she said. "It's like any time I see a film or a television show that has to deal with anything tragic, or hard or difficult between a mother and a child… I used to just watch it and feel empathy, and now… the crying begins."

The actress, who hits theaters next weekend opposite Ashton Kutcher in "Killers," added that her tears have become so bountiful that it has left husband Josh perplexed.

"I try to not cry because Josh just mercilessly makes fun of me. So then I get that headache from not crying. Holding it in is hard," she laughed.